Phosgene is employed in many fields of chemistry, either as auxiliary or as intermediate. The largest field of use in terms of quantity is the preparation of diisocyanates as starting materials for polyurethane chemistry. Particular mention may be made in this respect of the materials tolylene 2,4- and 2,6-diisocyanate (TDI), the isomers and homologues of diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI).
The industrial preparation of phosgene from carbon monoxide and chlorine is known from the prior art (e.g. Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. Vol. A 19p 413f., VCH Verlagsgesellschaft mbH, Weinheim, 1991). Here, carbon monoxide is combined in a stoichiometric excess with chlorine and passed over a fixed-bed catalyst. For industrial purposes, activated carbon is used as catalyst, and to this day the selection of a suitable activated carbon is carried out empirically (Mitchell et al.: Selection of carbon catalysts for the industrial manufacture of phosgene; Catal. Sci. Technol., 2012, 2, 2109-2115).
The activated carbon is consumed as time goes on and has to be renewed at regular intervals. Possible ways of regenerating the activated carbon catalyst have been proposed by E. Wygasch in “Ullmann's Encyklopädie der Technischen Chemie”, (Urban & Schwarzenberg, Munich, 13 (1962) 493), with the reactivation of the activated carbon being carried out at temperatures of from 550° C. to 630° C.
In the preparation of phosgene, the purity of the starting materials carbon monoxide and chlorine has to meet demanding requirements for reasons of plant safety and for reasons of product quality. The starting materials should have low contents of methane and hydrogen since these can lead to a strongly exothermic reaction when combined with chlorine. The temperature rise can lead to a dangerous reaction between chlorine and the material of the apparatus, known as a chlorine-iron fire.
The starting materials should have low contents of sulfur, bromine and iodine since these can remain in the phosgene produced and can lead to a deterioration in quality when the phosgene is used in a downstream process such as the preparation of isocyanates. Such a deterioration in quality is, for example, a poorer color of the end product.
Processes for preparing phosgene having a low content of by-products are known from the prior art. For example, contents of less than 150 ppm of carbon tetrachloride are achieved by means of suitable process conditions (EP 1 135 329 B1), or chlorine containing less than 50 ppm (EP 118 78 08 B1), or less than 400 ppm (EP 152 90 33 B1), of free or bound bromine or iodine is required as starting material.
EP 1 808 430 B1 discloses a process for preparing isocyanates, in which phosgene containing less than 100 ppm of sulfur in elemental or bound form is used.
The formation of phosgene is a strong exothermic reaction having an enthalpy of formation of −107.6 kJ/mol. Phosgene formed is subject to a dissociation equilibrium and decomposes back into the starting materials at elevated temperatures. At 100° C., phosgene dissociates to such an extent that it contains about 50 ppm of chlorine.
Residual contents of chlorine in the phosgene produced interfere in virtually all possible uses of this intermediate. In order to keep the chlorine content of the phosgene produced as low as possible, firstly carbon monoxide is used in excess and, secondly, the phosgene formation reaction is completed at the lowest possible temperatures.
For this reason, carbon monoxide is used in an excess of from 3% by volume to 10% by volume over the stoichiometrically required amount in the preparation of phosgene. The carbon monoxide used in excess cannot be recycled and represents a loss of material. For this reason, many efforts have been made in the past to develop more economical processes for preparing phosgene. Thus, EP 2 067 742 A1 describes a process for preparing phosgene with reduced CO emission or reduced CO losses by means of a main combining, a subsequent condensation of the phosgene and a subsequent after-combining of the residual gas with chlorine. A process having a regulating concept for minimizing the CO excess is presented in WO 2010/103029 A1.
A further important aspect in the preparation of phosgene is the safe and uniform removal of the heat of reaction. This is achieved, for example, by the reaction being carried out in a shell-and-tube reactor, known as the phosgene generator, in the tubes of which the activated carbon is located and around which reaction tubes a cooling medium is circulated by forced convection or natural convection and partially vaporized there. EP 0 134 506 B1 describes a process in which the cooling can be utilized for generating steam. In such shell-and-tube reactors, the phosgene formation reaction between carbon monoxide and chlorine over suitable activated carbon catalysts already proceeds at about 40° C.-50° C., with the temperature in the catalyst filling in the tubes being able to rise to about 600° C. and, depending on the intensity of the cooling employed, dropping back to 40° C.-150° C. by the reactor outlet.
Cooling the catalyst bed to below 100° C. is also known, since phosgene having residual contents of chlorine of less than 50 ppm can then be obtained. For many fields of application, for example the production of isocyanate for polyurethane production, such a chlorine content represents the upper limit of the specification since otherwise deterioration in quality can occur. This is described, for example, in EP 0 134 506 B1. Such a deterioration in quality can be, for example, an increased content of chlorinated by-products or a poorer color of the end product.
To avoid such a deterioration in quality in isocyanate production, it is therefore necessary to avoid both the stoichiometric excess of carbon monoxide and also elevated reactor exit temperatures at the phosgene generator at all times.
Although the above-described processes of the prior art are able to produce a phosgene which does not lead to any deterioration in quality of the end products, only processes in normal operation are described. Start-up processes until a stable operating state at the desired load (known as “start-up time”) or running-down processes until complete shutdown is attained (known as “running-down time”) are not taken into account. However, studies which have led to the present invention have shown that increased amounts of chlorine can be formed during the start-up and running-down time, and these get into the product stream.